Data symbol reading system

ABSTRACT

A stationary data symbol reader includes a CCD, lens, CCD drive circuit, amplification circuit, binarization circuit, memory, CPU, light sources, light source drive circuit, communication driver, switch circuit and trigger switch. When reading, a frame image capture is carried out, and a number of picture elements of the fields of the frame corresponding to the displacement of an authenticator pattern (associated with the data symbol) between a second field image and a first field image is calculated. The moving speed of the data symbol is calculated from the displacement. The optimal light exposure time is calculated from the moving speed, and the optimal intensity of illumination is calculated from the exposure time.

This is a division of application Ser. No. 08/666,863, filed Jun. 19,1996 now U.S. Pat. No. 5,754,670, the contents of which are hereinincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a data symbol reader for readinginformation encoded in a data symbol, for example, two-dimensionaltessellated data symbols.

Bar codes and associated devices for reading bar code information arewidely used for applications such as point-of-sale (POS) systems.However, most common bar codes and readers use the familiarone-dimensional format, in which information is encoded in aone-dimensional arrangement of parallel bars. A one-dimensional formatdata symbol can hold only a small amount of information.

Two dimensional symbols using a binary-encoded mosaic (or "tessellated")rectangular pattern, typically black and white, hold more information.Data symbol readers for reading the tessellated symbols use either anarea image sensor or a line image sensor. With a line sensor, thereading device and data symbol are moved relative to each other forauxiliary scanning, and the line sensor repetitively main scans lines ofthe data symbol to compose an area image. With an area sensor (usuallyan area CCD) an image of the data symbol pattern is captured in twodimensions simultaneously. Reading devices which use an image sensingdevice (i.e., an area image sensor) do not require relative movementbetween the reading unit and the data symbol for sub-scanning, and arethus able to read data symbols in a shorter time.

A stationary data symbol reader may be used for reading a moving datasymbol. One example of an appropriate application is reading datasymbols borne by products moving along an inspection or conveying linein a factory, either automatically or by hand.

When the data symbol reader is stationary, but the symbol itself moves,the moving speed of the symbol varies. If automatic, the speed will varybetween conveying devices. If an operator manually moves the itembearing the symbol past the symbol reader, the speed will constantlyvary. The conventional symbol reader cannot obtain the moving speed of adata symbol to be read.

In a case where a moving data symbol is read by a conventional datasymbol reader having an image sensing device, it is very difficult toaccurately set an exposure (exposure time, light amount) suiting themoving speed of the data symbol. If the exposure time is too long, theimage blurs and becomes unreadable. If the exposure time is too short,sufficient imaging information is not extracted, (for example,insufficient levels of contrast for thresholding,) leading to readingerrors.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide animproved data symbol reading system capable of reliably reading datasymbols moving at different speeds.

The present invention is directed to a data symbol reading system thatsatisfies this object. According to one aspect of the present invention,a data symbol reading system includes: a reading unit including an imagesensing device for capturing images and an optical system for formingimages on the image sensing device; and speed detecting means fordetecting a moving speed of the data symbol relative to the readingunit.

In this manner, the data symbol reading system is capable of detectingthe moving speed of a data symbol reader by the reading unit.

In one particular embodiment of the invention, the speed detecting meansincludes a controller for controlling the image sensing device to carryout two successive image captures with a predetermined time intervaltherebetween. A memory stores data corresponding to the two successiveimage captures. Calculation means calculates a displacement between thetwo successive image captures, and further calculates a moving speed ofthe data symbol based on the displacement and the predetermined timeinterval.

Accordingly, using the image sensing device itself, successive imagesare compared, and the relative displacement of the images is convertedinto a moving speed of the data symbol using a predetermined timeinterval between successive images. No additional sensors ordisplacement meters are necessary.

In this embodiment, the data symbol is preferably provided with anauthenticator pattern that moves together with the data symbol. Thecontroller controls the image sensing device to capture two successiveimages, each successive image capture including an image of theauthenticator pattern. The calculation means calculates a displacementof the image of the authenticator pattern between the two successiveimage captures, and calculates the moving speed of the data symbol basedon the displacement of the image of the authenticator pattern and thepredetermined time interval.

Accordingly, the authenticator pattern provides an indicator to thecalculation means that the image displacement between images having theauthenticator pattern therein should be calculated. Furthermore, sincethe authenticator pattern is provided separately from the data symbolitself, the authenticator pattern can be a very simple pattern, easilyrecognizable by the calculation means.

In an alternative embodiment, the image sensing device captures videoframe images, each video frame image having an odd line video field andan even line video field with a predetermined time intervaltherebetween. The speed detecting means includes a controller forcontrolling the image sensing device to carry out a video frame imagecapture. A memory stores data corresponding to the odd line video fieldand the even line video field. Calculation means calculates adisplacement between the odd line video field and the even line videofield, and calculates the moving speed of the data symbol based on thedisplacement and the predetermined time interval.

In this case, the two fields of a video frame, having a known timeinterval therebetween, are used for calculating the displacement of thedata symbol during the known time interval. The authenticator pattern asabove can also be used with this embodiment.

Preferably, this aspect of the invention includes exposure time settingmeans for setting an exposure time of the image sensing device based onthe calculated moving speed. Thus, the appropriate exposure time can beset according to the moving speed of the data symbol, preventingblurring of the symbol image or underexposure thereof.

In this case, the data symbol reading system may further include avariable intensity light source, and light intensity setting means forsetting the light emitting intensity of the light source based on theexposure time. Accordingly, not only can the exposure time be set toaccurately capture the data symbol, but if the required exposure time istoo short for the existing light amount, the variable intensity lightsource can be adjusted according to the exposure time to provide acorrect exposure, i.e., shutter-priority exposure control isimplemented.

According to another aspect of the present invention, a data symbolreading system, includes: a data symbol reader including an imagesensing device for capturing images, an optical system for formingimages of a symbol reading area on the image sensing device, and speeddetecting means for detecting a moving speed of the data symbol relativeto the reading unit. A moving subject bearing a data symbol is movablesuch that the data symbol moves through the symbol reading area.

Preferably, an authenticator pattern is provided on the moving subject.Further preferably, the authenticator pattern is provided on the movingsubject ahead of the data symbol in a moving direction of the movingsubject.

Accordingly, the authenticator pattern provides an indication to thespeed detection means that the speed of the moving subject, andtherefore of the data symbol, should be detected. Furthermore, since theauthenticator pattern is provided separately from the data symbolitself, the authenticator pattern can be a very simple pattern, easilyrecognizable by the speed detecting means.

According to a preferred arrangement, the speed detecting means includesa controller for controlling the image sensing device to capture twosuccessive images of the symbol reading area separated by apredetermined time interval, each successive image including an image ofthe authenticator pattern. A memory stores data representative of thetwo successive images. Calculation means calculates a displacementbetween the images of the authenticator pattern between the twosuccessive images, and calculates the moving speed based on thedisplacement the predetermined time interval.

In one development of this aspect of the invention, the data symbolreading system includes synchronization means for synchronizing movementof the moving subject with the image capture by the image sensingdevice. In this manner, the reading unit can recognize when the datasymbol image should be captured. In this case, the synchronization meansmay include a trigger switch connected to the reading unit, positionedin the path of the moving subject, and actuatable by the moving subject;and a timer for counting a predetermined delay after an actuation of thetrigger switch by the moving subject, the reading device responding tothe timer by starting the image capture.

In still another aspect of the present invention, a data symbol readingsystem includes: a reading unit including an image sensing device forcapturing images and an optical system for forming images on the imagesensing device. The image sensing device captures video frame images,each video frame image having an odd line video field and an even linevideo field with a predetermined time interval therebetween. Acontroller controls the image sensing device to carry out the videoframe image capture, while a memory stores data corresponding to the oddline video field and the even line video field. Calculation meanscalculates a displacement between the odd line video field and the evenline video field, and calculates the moving speed of the data symbolbased on the displacement and the predetermined time interval.

According to yet another aspect of the present invention, a data symbolreading system includes: a reading unit including an image sensingdevice for capturing images and an optical system for forming images onthe image sensing device; speed detecting means for detecting a movingspeed of the data symbol relative to the reading unit; and exposure timesetting means for setting an exposure time of the image sensing devicebased on the calculated moving speed. In this case, the data symbolreading system preferably further includes a variable intensity lightsource, and light intensity setting means for setting the light emittingintensity of the light source based on the exposure time.

According to yet still another aspect of the present invention, a datasymbol reading system includes: a reading unit including an imagesensing device for capturing images and an optical system for formingimages on the image sensing device. A controller controls the imagesensing device to capture two successive images with a predeterminedtime interval therebetween. The data symbol is provided with anauthenticator pattern that moves together with the data symbol, and eachsuccessive image capture includes an image of the authenticator pattern.A memory stores data corresponding to the two successive image captures.Calculation means calculates a displacement of the image of theauthenticator pattern between the two successive image captures, andcalculates the moving speed of the data symbol based on the displacementof the image of the authenticator pattern and the predetermined timeinterval.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a preferred embodiment of the datasymbol reading system according to the invention;

FIG. 2 is a bottom plan view of the data symbol reading system shown inFIG. 1;

FIG. 3 is block diagram showing a circuit configuration of the datasymbol reading system shown in FIG. 1;

FIG. 4 is a timing chart describing a frame image capture;

FIGS. 5A though 5D are schematic plan views--; and a moving subject anda data symbol reading area;

FIGS. 6A and 6B are explanatory diagrams showing an acquired first fieldimage and an acquired second field image in a memory;

FIG. 7 is a graph showing a relationship between moving speeds of asymbol and exposure times;

FIG. 8 is a graph showing a relationship between light exposure timesand light intensities; and

FIG. 9 is a flow chart showing the control of the data symbol readingsystem shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a perspective view of a preferred embodiment of a data symbolreading system according to the invention, FIG. 2 is a bottom plan viewof the data symbol reader of FIG. 1, and FIG. 3 is a block diagramshowing a circuit configuration of the data symbol reader illustrated inFIG. 1.

With reference to FIGS. 1 through 3, the data symbol reading systemaccording to the invention includes a data symbol reader 1 and a movingsubject 9. The data symbol reader 1 of the invention is stationary,i.e., mounted on a stand or positioning arm. The data symbol reader 1includes a casing 2 having a head portion 22 at the reading end thereof.A rectangular reading end opening 31 (shown in FIG. 2) is formed at thereading end of the head portion 22.

The casing 2 houses a reading unit 4, and a control circuit 50. As shownin FIG. 3, the control circuit 50 includes a signal processing circuit5, a light source driving circuit 42, and a communication driver 16.

The reading unit 4 includes two light sources 41, 41; an area chargecoupled device (CCD) 43 as an image sensing device and; an opticalsystem 44. The optical system 44 guides light from a symbol reading area36 to form an image on the light receiving surface of the CCD 43. Thesymbol reading area 36 is defined by the reading unit 4, and a datasymbol 38 borne by a moving subject 9 passes through the symbol readingarea 36.

The two light sources 41 illuminate the symbol reading area 36, and areset in a substantially symmetrical manner with respect to the light path47. A light emitting element such as an LED, a halogen lamp, asemiconductor laser, or a light valve is used as the light source 41. Adiffuser plate (not shown) with a rough or translucent surface may beinstalled at the light emitting side of the light sources 41, in orderto make the luminance at the symbol reading area 36 more uniform. Thelight sources 41 are controlled by the light source driving circuit 42in the control circuit 50.

The CCD 43 has many photodiode light receiving (or "picture") elementsdisposed in an array. Each of the picture elements accumulates anelectrical charge corresponding to the amount of light received, andtransfers the charge in sequence at a predetermined time. Thetransferred charges are readable (analog) image signals. Furthermore theCCD has an electronic shutter function for controlling the time ofelectric charge accumulation.

The symbol reading area 36 is formed on a reference plane 37 on themoving subject 9, i.e., the surface upon which the data symbol 38 ispositioned. The light reflected from the symbol reading area 36 isdirected to the CCD 43 by the optical system 44. The symbol reading area36 is aligned with the front opening 31 of the head part 22, and isincluded in the effective sensing area of the CCD 43.

As shown in FIG. 2, the data symbol 38 consists of a tessellated patternof black and white cells (alternatively, black and transparent cells)arrange din an array. The array has no less than 2 columns (i.e., xcolumns, where x>1) and rows (i.e., y rows, where y>1) of cells. In thisway, each cell can express 0 or 1 in a binary system, and informationcan be specified by a binary combination of cells. However, the datasymbol 38 readable by the data symbol reader 1 is not restricted to theillustrated arrangement. The control circuit 50 of the data symbolreader 1 includes a signal processing circuit 5 for processing the imagesignals received from the reading unit 4. As shown in FIG. 3, the signalprocessing circuit 5 includes a CCD driving circuit 6, an amplificationcircuit 8, a binarization circuit 10, a memory 12, and a centralprocessing unit (CPU) 15 as control means.

In operation, the light source driving circuit 42 activates the twolight sources 41 to illuminate the symbol reading area 36. Reflectedlight from the symbol reading area 36 passes through the optical system44, forming an image on the light receiving surface of the CCD 43. TheCCD 43 then outputs image signals (analog signals) corresponding to theamount of light received, which are amplified and undergo A/D conversionat the amplification circuit 8, binarized at the binarization circuit10, read by the CPU 15, and stored in the memory 12. The light sourcedrive circuit 42, under the control of the CPU 15, supplies a variableamount of power for a variable duration to the light sources 41, therebyturning the light sources 41 ON and OFF, and further controlling thelight intensity generated. The length of exposure and intensity arethereby controlled by the CPU 15.

The CPU 15 is further connected to the light source driving circuit 42,the communication driver 16, and a switch circuit 13 (for a triggerswitch 14). In addition, the CPU has a table memory 15a for storingexposure tables for intensity and duration of exposure, and a timer 15bfor timing the exposure. Optionally, an LED, LCD, or CRT display (notshown) is connected to the CPU 15.

The trigger switch 14 has an actuating lever 14a (shown in FIGS. 5Athrough 5D) and is turned ON when the actuating lever 14a is pressed.The trigger switch 14 is positioned such that the actuating lever 14a ispressed by the side edge portion of the moving subject 9 when the movingsubject 9 reaches a predetermined position at which the capture ofpreparatory field images for speed detection is started.

This preparatory image capture position is a position of the movingsubject 9 when an authenticator pattern 91 on the moving subject 9reaches the symbol reading area 36 (in this embodiment, when theauthenticator pattern 91 is in the vicinity of the center of the symbolreading area 36). The trigger switch 14 thus constitutes synchronizationmeans for synchronizing the movement of the moving subject 9 with theimage capture timing by the CCD 43.

The data symbol reader 1 is positioned such that the light received fromthe symbol reading area 36 is formed as an image on the CCD 43 by theoptical system 44 and so that the reading and opening 31 is parallel tothe reference plane 37.

Furthermore, as shown in FIG. 1, a conveyor (not shown) which transfersthe moving subject 9 bearing the data symbol 38 is positioned oppositethe data symbol reader 1 in FIG. 1. The conveyor transfers the movingsubject 9 parallel to the plane within which the picture elements of theCCD 43 are arrayed, and through the symbol reading area 36 opposite thehead portion 22.

In this embodiment, the authenticator pattern 91 is a band-like patternon the moving subject 9 extending perpendicular to the moving direction,provided in front of the data symbol 38 in the moving direction(downstream) thereof. The authenticator pattern 91 is separated from thedata symbol 38 by a predetermined distance.

Speed Detection

When the trigger switch 14 of the data symbol reader 1 is turned ON by apassing moving subject 9, a process to carry out preparatory field imagecaptures (i.e., light exposure and image capture) to detect the movingspeed of the data symbol 38 is begun, and the exposure is subsequentlydetermined.

The preparatory field image capture is carried out twice at apredetermined interval in order to detect the change in position of theauthenticator pattern 91, as reference for detecting the moving speed ofthe data symbol 38.

FIG. 4 is a timing chart showing timing for the CCD 43 to perform twofield image captures. The fields are two video fields separated at 60 Hzof a 30 Hz video frame, i.e., the first field has only odd lines and thesecond has only even lines of the CCD 43. The light exposure time(electric charge accumulation time) is controlled by the electronicshutter function of the CCD 43. When performing an image capture, thesecond field image capture is performed 1/60 second after the firstfield image capture.

For the preparatory first and second field image captures, the imagecapture is performed at the shortest light exposure time (i.e., thehighest "shutter speed") and with the maximum light emitting intensityof the light sources 41. The image data (image signals) are binarized,and the binarized data (binarized signals) are written in the memory 12.The binarized data of the first field image are written in a first fieldimage storing area of the memory 12, and the second field binarized dataare written in a second field image storing area of the memory 12.

The displacement ΔD of the authenticator pattern is calculated as thedisplacement of a second authenticator pattern image 91_(i2) (shown inFIG. 6B) from a first authenticator pattern image 91_(i1) (shown in FIG.6A). The moving speed V of the data symbol 38 is calculated on the basisof the displacement of the authenticator pattern 91 and the timedifference (in this case, 1/60 second) between the first and secondfield image captures. The moving speed V is used to determine anappropriate exposure duration, an appropriate illumination intensity,and a waiting interval W until a data reading image capture isperformed. The waiting interval W corresponds to a time from the end ofthe speed detecting process until the data symbol 38 reaches apredetermined position (for example, the center) within the symbolreading portion 36, and is easily determined from the moving speed V anda standard distance between the authenticator pattern 91 and the symbol38.

FIGS. 5A through 5D are schematic plan views showing positions of themoving subject 9 as it moves through the symbol reading area 36. FIGS.6A and 6B are explanatory views showing the first field image and secondfield image (the first acquired preparatory field image and the secondacquired preparatory field image) as stored in the memory 12.

As shown in FIG. 5A, if the authenticator pattern 91 is not located inthe data symbol reading area 36 when images are captured, binarized datahaving an authenticator pattern 91 is written in the memory 12. In thepreferred embodiment, since the trigger switch 14 is turned OFF in thestate shown in FIG. 5A, no image capture at all is carried out.

When the authenticator pattern 91 is positioned in the data symbolreading area 36 (as shown in FIGS. 5B and 5C), field images includingthe authenticator pattern 91 are written in the memory 12, as shown inFIGS. 6A and 6B. The first preparatory acquired field image shown inFIG. 6A corresponds to the image captured in FIG. 5B, and the secondpreparatory acquired field image shown in FIG. 6B corresponds to theimage captured in FIG. 5C.

To find the displacement of the authenticator pattern 91, first thepresence of an authenticator pattern 91 is checked in the first field. Apredetermined nth (odd) horizontal line (line H_(n) in FIG. 6A) of thefirst field image in the memory 12 is checked by the CPU 15, and if anidentifiable authenticator pattern image 91_(i1) is found, the positionis recorded. For example, the leading edge 92_(i1) and width of anauthenticator pattern image 91_(i1) are searched for and identified, ifpresent. Once the authenticator pattern image 91_(i1) is identified, adistance D1 from the leading edge 92_(i1) to the leading edge of theentire first field image is found. The distance D1 is preferablyexpressed as a number of picture elements.

If an authenticator pattern is found, the positions of the first andsecond preparatory field images are compared. That is, the position ofthe authenticator pattern 91 image is analyzed with respect to the imageboundaries.

For the second field image, a predetermined (even) horizontal line (forexample, line H_(n+1) in FIG. 6B) of the second field image in thememory 12 is checked by the CPU 15, and the position is recorded. Forexample, the leading edge 92_(i2) and width of an authenticator patternimage 91_(i2) are searched for and identified. Once the authenticatorpattern image 91_(i2) is identified, a distance D2 from the leading edge92_(i2) to the leading edge of the entire second field image is found.The distance D2 is preferably expressed as a number of picture elements.

In order to calculate the speed of the moving subject 9 (and of the datasymbol 38), firstly, the displacement of the second field image from thefirst field image, i.e., the difference ΔD between D1 and D2, iscalculated. The moving speed V of the data symbol 38 relative to thereading portion is then calculated from an expression: ##EQU1##

In expression (1), ΔD is the difference between D1 and D2 expressed as anumber of picture elements, C_(H) is the length of one side of a pictureelement of the CCD 43, and M is the magnification of the optical system44. Furthermore, as shown in FIG. 4, the denominator of expression (1)is the image capture time interval between the first and secondpreparatory field image captures (that is, 1/60 second).

Exposure Calculations

The exposure tables stored in the CPU table memory 15a expressrelationships between moving speed V, exposure time, and illuminationintensity. FIG. 7 is a graph showing an example relationship between themoving speed V and an optimum exposure time Ts, and FIG. 8 is a graphshowing an example relationship between the optimum exposure time Ts andthe optimum illumination (optimal light intensity) for the exposure timeTs.

As shown in FIG. 7, the optimum exposure time Ts (adequate electriccharge accumulation time) can be calculated from the moving speed V. Aplurality of appropriate combinations of the moving speeds V and optimumexposure times Ts is stored in the table memory 15a of the CPU 15. Theoptimum exposure time Ts is calculated by setting a Ts corresponding tothe moving speed V.

As shown in FIG. 8, the optimum illumination (adequate light intensity)can be calculated from the exposure time Ts. A plurality of appropriatecombinations of optimum exposure times Ts and illumination levels isstored in the table memory 15a of the CPU 15. The optimum illuminationlevel is calculated by setting an illumination level corresponding tothe optimum exposure time Ts.

Data Reading

A data reading (symbol decoding) image capture is performed in order toread the data symbol 38 after the waiting interval W elapses. The datareading image capture uses a full frame video image having all of thepicture elements available.

In the data reading image capture, the CPU 15 further activates the CCDdriving circuit 6 after the waiting interval W elapses. A horizontal CCDdriving pulse and a vertical CCD driving pulse are output from the CCDdriving circuit 6 to the CCD 43 to control the accumulation and transferof charge at the CCD 43.

Clock signals are also generated at the CCD driving circuit 6. Forexample, composite clock signals, having a horizontal synchronizationsignal and a vertical synchronization signal combined with a clocksignal, are transmitted from the CCD driving circuit 6 to the CPU 15.

The amplification circuit 8 is connected to the CCD 43, and amplifiesthe (analog) image signals from the CCD 43, and performs A/D conversionto give a digital image signals (for example, 8-bit image signals). Thedigital image signals are then input into the binarization circuit 10.

In the binarization circuit 10, the digital image signals from eachpicture element are turned into a binary value of "1" or "0" accordingto predetermined threshold data. A binarized data value of "1"corresponds to a black part of the data symbol 38 while a value of "0"corresponds to a white part. The binarized data output from thebinarization circuit 10 are transferred through the CPU 15 And stored inpredetermined addresses in the memory 12 by means of an address counterincluded in the CPU 15. This address counter is driven according to thecomposite clock signals input from the CCD driving circuit 6.

In the reading operation, firstly, the binarized data are readsequentially from the memory 12 in accordance with the addressesdesignated by the address counter (the read order may be reversed withrespect to the storage order). The CPU 15 performs any necessary imageprocessing. For example, image inversion; extraction of only binarizeddata for the data symbol 38 based on the coordinate data of the borderpicture images; dropout correction; rotation, and the like, are thenperformed on the binarized data for one image. The CPU 15 also decodesthe binarized data into useable data in accordance with the decodingsystem for the particular type of the data symbol 38. The decoded datais then sent via the communication driver 16 to an externally connectedhost computer 17. The host computer 17 may be, for example, a personalcomputer or a work station. The storage, tabulation, and so on of theuseable data are then performed on the host computer 17.

Control Operations

FIG. 9 is a flow chart showing the control flow of the CPU 15 forcontrolling a data symbol reader 1 according to the invention.

When the trigger switch 14 is turned ON, the process of FIG. 9 isstarted. In step S101, a frame image capture is performed, and the imageof the first field (odd number field) and second field (even numberfield) are acquired. The first and second field image data (imagesignals) are binarized, and the binarized data (binarized signals) arewritten in the memory 12.

In step S102, the process analyzes the first field image, and checks fora (black) authenticator pattern, for example, for checking along the nthhorizontal line for an authenticator pattern of the proper width, aspreviously described. If an authenticator pattern 91 is detected, thenumber of CCD picture elements D1 corresponding to the distance from theleading end of the first field image to the leading edge 92 of theauthenticator pattern 91 (as shown in FIG. 7) is calculated. The numberof picture elements D1 is calculated from the memory 12 on the basis ofthe reading address of the edge 92.

In step S103, the process checks if an authenticator pattern 91 wasfound in step S 102. If an authenticator pattern 91 was not found (N instep S103), then the process loops back to step S101 and scans(acquires) a new image.

If, however, an authenticator pattern 91 was found in step S102, thenthe process proceeds to step S104. In step S104, the number of CCDpicture elements D2 corresponding to the distance from the leading endof the second field image to the edge 92 of the authenticator pattern 91is calculated, similarly to the procedure for the first field.

In step S105, the displacement of the authenticator pattern 91, that is,the difference ΔD between D1 and D2 is calculated.

In step S106, the moving speed V of the moving subject 9 and the datasymbol 38 is calculated according to the previously described expression(1).

In step S107, the optimal light exposure time Ts (adequate electriccharge accumulation time) is calculated from the moving speed V on thebasis of the table data (as described in FIG. 7) in the table memory15a.

In step S108, the optimal light intensity is calculated from the optimallight exposure time Ts on the basis of the table data (as described inFIG. 8) in the table memory 15a.

In step S109, the waiting time W until a normal image capture can betaken (i.e., for reading the symbol data as opposed to setting theexposure) is calculated from the moving speed V. The waiting time is setin the timer 15b incorporated in the CPU 15, and the timer 15b isstarted.

In step S110, the process loops in place until the waiting time W, ascounted by the timer 15b, is finished.

In step S111, a normal image capture (normal light exposure) is carriedout, using the previously determined optimal light exposure time Ts andillumination intensity.

In this case, as shown in FIG. 5D, since the appropriate waiting time Whas passed, the data symbol 39 is in the symbol reading area 36 when thenormal image capture is performed. The image data (image signals) arebinarized, and the binarized data (binarized signals) are written in thememory 12.

In the preferred embodiment, since both the light exposure time and theillumination light intensity are set to achieve an optimal lightexposure quantity, the normal image capture is carried out under thebest light exposure conditions at all times. However, it is acceptablethat only the light exposure time is controlled until a decoding erroroccurs, in which case the light intensity is then controlled.

In step S112, the previously described image processing and decoding arecarried out and the decode data is transmitted to the host computer 17via the communication driver 16.

Accordingly, in the embodiment of a data symbol reader 1 according tothe invention, the optimal light exposure time (also known as shutterspeed) is set according to the moving speed of the data symbol 38. Evenif the moving speed of data symbol 38 fluctuates, the embodiment of adata symbol reader 1 according to the invention prevents image blur fromoverly long exposures and lack of definition from overly shortexposures. In this manner, the data symbol reader 1 prevents readingerrors, and increases the reading accuracy of data symbols 38.

Furthermore, since the light emitting intensity of the light sources 41is adjusted according to the light exposure time, the data symbol readerprevents over or underexposure from inappropriate illumination.

Still further, since the exposure time and light intensity areautomatically set to the optimal values, the operation is more simplethan manually set devices.

Although a data symbol reader according to the invention is explainedwith reference to a preferred embodiment, other constructions arepossible without departing from the spirit or scope of the invention.

For example, although the speed detecting device detects the movingspeed of the data symbol 38 on the basis of displacement of theauthenticator pattern 91 between sequential field images having a knowndelay therebetween, the speed detecting device may alternatively beconstructed so that a predetermined delay is set, or so thatnon-sequential fields or frames are used.

The present disclosure relates to subject matter contained in JapanesePatent Application No. HEI 07-179533, filed on Jun. 21, 1995, which isexpressly incorporated herein by reference in its entirety.

What is claimed is:
 1. A data symbol reading system, comprising:areading unit including an image sensing device that captures an image ofa data symbol and an image of an authenticator pattern, and an opticalsystem that forms an image of the data symbol and an image of theauthenticator pattern on the image sensing device, the authenticatorpattern being provided at a predetermined distance from the data symbol,the image sensing device capturing the image of the authenticatorpattern prior to capturing of the image of the data symbol; a speeddetecting device that detects a moving speed of the data symbol relativeto the reading unit in accordance with the authenticator patterncaptured by the image sensing device prior to capturing of the image thedata symbol; a time interval calculator that calculates a waitinginterval in accordance with the predetermined distance and the movingspeed detected by the speed detecting device, the waiting interval beinga time from the end of the detection of the moving speed by the speeddetecting device until the data symbol reaches a predetermined positionto be captured; a table memory that stores relationship between aplurality of moving speeds and a plurality of exposure times; and anexposure time setting device that retrieves one of the plurality ofexposure times corresponding to the calculated moving speed of the datasymbol relative to the reading unit, so as to be set an optimum exposuretime of the image sensing device to read the data symbol, the readingunit starting capture of the image of the data symbol using the exposuretime set by the exposure time setting device upon elapse of the waitinginterval.
 2. The data symbol reading system according to claim 1,further comprising:a variable intensity light source; and lightintensity setting means for setting the light emitting intensity of thelight source based on said exposure time.
 3. The data symbol readingsystem according to claim 1, whereinthe speed detecting device capturesa first image of the authenticator pattern and captures a second imageof the authenticator pattern a predetermined time after capturing of thefirst image, said speed detecting device detecting the displacement ofthe authenticator pattern between the capture of the first image and thecapture of the second image, and detects the moving speed in accordancewith the detected displacement.
 4. The data symbol reading systemaccording to claim 1, wherein the image sensing device captures theimage of the authenticator pattern at a shortest exposure time.